Hong Seok Lee

Discrete states and carrier-phonon scattering in quantum dot population dynamics

The influence of the growth conditions of multilayer CdTe/ZnTe quantum dots (QDs) on Si substrate upon their carrier dynamics is studied using intensity integration and broadening photoluminescence. The unusual temperature dependence of the line broadening is explained using a model for interband transitions that involves a lowest discrete electronic state (1Se) with different discrete hole states (1S3/2 and 2S3/2) and a 1P transition. These transitions are expected to play a critical role in both the thermally activated energy and the line broadening of the QDs. We also demonstrate that a thermally activated transition between two different states occurs with band low-temperature quenching, with values separated by 5.8–16 meV. The main nonradiative process is thermal escape assisted by carrier scattering via emission of longitudinal phonons through the hole states at high temperature, with an average energy of 19.3–20.2 meV.

Enhancement of light output power in GaN-based light-emitting diodes using hydrothermally grown ZnO micro-walls

We report on the efficiency enhancement in GaN-based light-emitting diodes (LEDs) using ZnO micro-walls grown by a hydrothermal method. The formation of ZnO micro-walls at the indium tin oxide (ITO) border on the LED structure is explained by the heterogeneous nucleation effect. The light output power of LEDs with ZnO micro-walls operated at 20 mA was found to increase by approximately 30% compared to conventional LEDs. Moreover, the finding of nearly the same current-voltage characteristics of GaN-based LEDs with and without a ZnO micro-wall shows that the ZnO micro-wall does not influence the electrical properties of the device but only leads to an increase in the light extraction efficiency. From the confocal scanning electroluminescence results, we confirm that ZnO micro-walls enhance the light output power via the photon wave-guiding effect.

Submicro-electroluminescence spectroscopy of dodecagonal micropit-arranged blue light-emitting diodes

We investigate the local electroluminescence (EL) characteristics of c-plane blue InGaN/GaN light-emitting diodes (LEDs) with periodically arranged dodecagonal micropits (DMs) which have {1?0??1??1} and {1?1??2??2} facets using confocal scanning electroluminescence microscopy (CSEM). By increasing the injection current from 1 to 20?mA, the EL peak for c-plane LEDs is shifted to a shorter wavelength (by up to 5.5?nm), but the EL peaks for semi-polar {1?0??1??1} and {1?1??2??2} facets are nearly unchanged because of the substantially reduced polarization-related electric fields in the quantum wells. CSEM images show various emitting distributions due to the different emission origins as a function of peak position in the EL spectrum. The present observations can help one to improve the understanding of the spatial origin of increased external quantum efficiency in InGaN LEDs with DM arrays.

Optical characterization of nanomaterials

1 Department of Energy Science, Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea 2Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA 23529, USA 3 School of Mechanical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea 4Department of Physics, Yeungnam University, Gyeongsan 712-749, Republic of Korea 5 Department of Physics, Jeju National University, Jeju 690-756, Republic of Korea

Interband Transition and Confinement of Charge Carriers in CdS and CdS/CdSe Quantum Dots

Quantum-confined nanostructures open up additional perspectives in engineering materials with different electronic and optical properties. We have fabricated unique cation-exchanged CdS and CdS/CdSe quantum dots and measured their first four exciton transitions. We demonstrate that the relationship between electronic transitions and charge-carrier distributions is generalized for a broad range of core-shell nanostructures. These nanostructures can be used to further improve the performance in the fields of bio-imaging, light-emitting devices, photovoltaics, and quantum computing.